<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>select演示文档</title>
    <style>
      div {
        border: 1px solid blue;
        display: flex;
        flex-flow: row wrap;
        justify-content: center;
      }
    </style>
  </head>
  <body>
    <div>
      <form action="">
        <select name="select" id="select" size="1" multiple>
          <option value="opt1" selected>--请选择--</option>
    <optgroup label="第一组">
        <option value="opt2">选项一</option>
        <option value="opt3">选项二</option>
        <option value="opt4">选项三</option>
        <option value="opt5">选项四</option>
    </optgroup>
    <optgroup label="第二组">
        <option value="opt2">选项一</option>
        <option value="opt3">选项二</option>
        <option value="opt4">选项三</option>
        <option value="opt5">选项四</option>
    </optgroup>
      </form>
    </div>
  </body>
</html>
